CDK4 and yolk sac tumor: We also confirmed CDK4 expression on protein level by immunohistochemistry of formalin-fixed-paraffin-embedded GCT tissues and found mainly cytoplasmatic, but also nuclear staining in seminomas (n = 8), ECs (n = 7), yolk-sac tumours (n = 4) and teratomas (n = 3) (Fig. 1c).